A lightweight and high-strength material with high durability has been required as an airframe material used for the main wing, the integral tank, and the like of an aircraft. For example, a lightweight metal material, such as an aluminum alloy, is used as an airframe material. In addition, with a recent increasing demand for these, a carbon fiber composite material composed of a resin material reinforced with carbon fibers has been used as an airframe material.
When another member is bonded to a carbon fiber composite material, it is preferable to polish, with an abrasive, a bonding region of the surface of the carbon fiber composite material to which the other member is to be bonded, to reduce asperities, thereby increasing the adhesion between the carbon fiber composite material and the other member. In this case, to ensure the reduction of asperities of the bonding region to which the other member is to be bonded, it is necessary to polish a region larger than the bonding region.
However, if a region larger than the bonding region is polished, a polishing region which has been polished with an abrasive after the other member is bonded to the surface of the carbon fiber composite material is partly exposed as a surface of the carbon fiber composite material.
If the surface of the carbon fiber composite material is excessively polished with an abrasive, carbon fibers are partly exposed on the surface of the carbon fiber composite material.
In this case, if lightning strikes the carbon fiber composite material, carbon fibers exposed on the surface of the carbon fiber composite material may discharge upon a dielectric breakdown, thereby causing a spark (edge glow phenomenon).
For this reason, when the surface of the carbon fiber composite material is polished with an abrasive, it is preferable to avoid exposure of carbon fibers on the surface of the carbon fiber composite material.
Japanese Unexamined Patent Application, Publication No. 2008-55770 discloses forming a second resin layer below a first resin layer formed on the surface of a fiber reinforcing resin composite material with a color different from that of the first resin layer. According to JP 2008-55770, visual checking of a change in the color of the particulate resin abrasive suppresses polishing of fibers.
However, according to JP 2008-55770, if visual checking is not accurately performed, the second resin layer is polished and fibers are exposed on the surface.
Besides, even with visual checking, there is a risk of a failure in checking a change in the color of the particulate resin abrasive due to the illuminance or the like in the environment of polishing work, so that the second resin layer may be polished and fibers may be exposed on the surface.